Owing to the advancement in digital appliances, recently, capacitors having a low impedance and an excellent high frequency characteristic even in a high frequency region are strongly demanded. To meet such market needs, capacitors using conductive high polymers obtained by polymerizing pyrrole, thiophene or aniline as the electrolyte are being developed and commercially produced.
Hitherto, a solid electrolytic capacitor of this kind comprises, as disclosed in Japanese Laid-open Patent No. 63-158829, an anode electrode made of a valve metal having a dielectric oxide layer, the dielectric oxide layer formed on this anode electrode, a conductive high polymer layer formed by pyrolysis of manganese dioxide layer on this dielectric oxide layer, and a cathode electrode placed on this conductive high polymer layer. The conductive high polymer layer is formed by electrolytic process in an electrolytic polymerization solution using manganese dioxide layer as the anode.
Formation of conductive high polymer layer by this electrolytic polymerization is quick in forming a conductive high polymer layer as compared with chemical polymerization or vapor phase polymerization, and it requires a relatively simple equipment, and hence it is said to be beneficial for industrial production.
In the prior art, however, the forming condition of manganese dioxide layer has great effects on the principal characteristics of the solid electrolytic capacitor such as capacitance, tan .delta. and impedance.
That is, when the anode electrode is formed by a method of bonding particles of valve metal into a porous substance by sintering, or a method of multiple etching pits by etching process, such anode electrode has an undulated surface of an expanded surface area of fine pores or the like. The dielectric oxide layer formed on the surface of the anode electrode having fine pores has multiple fine pores and exposed portions reaching the inner depth. In the prior art, the manganese dioxide layer formed on this dielectric oxide layer is formed only on the exposed portions, and not formed in the inner parts of the fine pores. The conductive high polymer layer is formed only on this manganese dioxide layer. That is, in such conventional solid electrolytic capacitor, it was possible to have a cavity in the inside. In such conventional constitution, when the anode electrode has an expanded undulated surface, sufficient capacitance and sufficient impedance corresponding to the expanded undulated surface could not be obtained. Thus, there were serious problems also in the constitution of using conductive high polymer as the electrolyte.
It is hence an object of the invention to present a solid electrolytic capacitor exhibiting a desired effect sufficiently, in the solid electrolytic capacitor using an anode electrode having an undulated surface and a conductive high polymer as electrolyte, and a method of manufacturing the same.